1. Field of the Invention
The present invention relates to a quench detection system for a superconducting fault current limiter and to means for quench detection in a superconducting fault current limiter.
2. Description of the Related Art
A fault current limiter is a device which automatically limits fault currents in electrical distribution or transmission networks, in particular high voltage networks, to a low current value close to the nominal current. The benefit of such a device is that it reduces drastically the short circuit power of the high voltage network and, thus, allows to interconnect networks without increasing the short circuit power or to decrease safety margins so that other machineries connected to the network can be designed for lower short circuit power and, therefore, can be made lighter and cheaper.
Superconductors, especially high-temperature superconductors, are well suited for use in a fault current limiter due to their property to lose superconductivity and transit from the non-resistive superconducting state to a normal state with high electric resistivity when at least one of the critical current (Ic), the critical temperature (Tc) or the critical magnetic field (Hc) of the superconductor material is exceeded. This transition from the superconducting state to the normal resistive state is referred to “quenching”.
In normal operation with nominal current In, that is, in the cooled state, the superconductor material is in its superconducting state with essentially zero resistance so that there is essentially no voltage over the whole fault current limiter—the fault current limiter is “invisible” for the network. In case of short circuit fault current the current rises to several times the nominal current In exceeding Ic of the superconductor material which causes the superconducting material to transit to the normal resistive state with generation of high voltage. That is, in normal operation in the superconducting state essentially no voltage difference is observed at both ends of the fault current limiter, whereas in fault condition a large voltage difference is measured due to increasing electrical resistance.
In quenching the superconductor material has to absorb a large quantity of energy and, in the consequence, is heated up.
In order to avoid overheating and damage of the fault current limiter current flow through the fault current limiter must be interrupted within a limited period of time. Since the process of quenching proceeds within a very short time of only some tens of milliseconds quench detection must be very fast. Thus, means for quench detection are provided which, on quench, send a fault signal to a circuit breaker for switching off current flow.
In the prior art various method for quench detection are known.
For example, EP 0 828 331 relates to a high temperature superconducting coil assembly with individual coils being connected in series. At both end portions of the coil assembly a flux loop is provided. In case of quench an unbalance occurring in the flux loops is detected and serves to open an off-switch.
U.S. Pat. No. 5,999,383 relates to the detection of quench by measuring voltage between the superconducting state with about no voltage difference and the resistive state with high voltage difference at both ends of the current limiter. For facilitating the detection of voltage difference any “noise” caused by inductive voltage is compensated by a specific design.
Also JP 59-152604 relates to the detection of quench by measuring voltage difference. As in U.S. Pat. No. 5,999,383 referred to above “noise” caused by induced voltage is eliminated by a specific design.
In JP 2009-206237 use is made of voltage difference for detecting quench. The voltage difference measured is Fourier transformed for determining whether or not quenching has occurred.